Understanding the H3N2 flu strain
Medically reviewed by Dr. Betsy Koickel, MD on January 22nd, 2026.
The 2025-2026 flu season has been declared “moderately severe” by the Centers for Disease Control and Infection. This is because of the currently circulating new flu strain called H3N2 influenza.
To protect yourself and your family from the flu this year, it is important to understand what this strain is, who is most at risk, and what steps you can take to stay healthy this year.
What is the H3N2 flu strain?
The H3N2 flu strain is a subtype of the influenza A virus. It is identified by its surface proteins hemagglutinin (H3) and neuraminidase (N2). Currently, it is one of the main causes of seasonal flu worldwide.
How H3N2 differs from other flu strains
H3N2 is different from previous strains like H1N1. This particular flu strain mutates faster than many other influenza strains, allowing it to evade immunity from past infections or vaccination. It also infects the lower respiratory tract more efficiently and affects older adults more than other groups, leading to higher rates of hospitalization and complications.
The 2025-2026 flu season and H3N2 activity
The 2025-2026 flu season has seen H3N2 dominating circulation, accounting for the vast majority of cases across the globe. This particular strain is spreading rapidly, resulting in higher levels of hospitalization and pediatric deaths when compared to other flu seasons.
Surveillance data indicate that the H3N2 influenza strain is experiencing activity at some of the highest levels in decades. The underlying cause may be a partial mismatch between circulating H3N2 influenza strains and the vaccine. Even if it’s not a perfect match this year, vaccination remains the primary tool to reduce severe disease and hospitalizations.
Common symptoms of H3N2 infection
The H3N2 flu strain symptoms are similar to previous strains, infecting the respiratory system. Symptoms may include:
- Cough (often persistent or worsening)
- Fatigue and weakness
- Headache
- Loss of appetite
- Muscle and body aches
- Nausea, vomiting, or diarrhea (more common in children)
- Runny or stuffy nose
- Shortness of breath or chest discomfort
- Sore throat
- Sudden onset of fever and chills
Who is most at risk?
People most at risk for severe H3N2 infection include older adults, particularly those 65 or older, young children, pregnant women, and those with chronic health conditions. These groups are more likely to experience complications like pneumonia or hospitalization because H3N2 tends to cause more severe illness in those with weakened or aging immune systems.
Vaccine effectiveness and protection
Flu vaccines are updated annually to match predicted circulating strains. But this year, because the H3N2 flu strain mutates so rapidly, vaccine effectiveness has been lower than anticipated.
Even if it’s not perfect, getting your flu shot remains important because it significantly reduces the risk of severe illness, hospitalization, and death, especially in older adults and other high-risk groups.
Prevention and safety measures
If you are trying to avoid the flu or severe illness this year, there is a lot you can do to help your family and community stay healthy. A few prevention measures include:
- Consider masks in crowded or high‑risk settings
- Cover coughs and sneezes
- Improve indoor ventilation when possible
- Get vaccinated each year
- Seek early treatment, especially if high risk
- Stay home when sick
- Wash your hands often
While you can’t prevent every illness, practicing these safety measures can help lower your risk of severe infection.
Treatment options
For most people, the primary treatment option for the new flu strain will be supportive care, such as hydration and rest. Antiviral medications, such as oseltamivir (Tamiflu), can be effective when started within the first 48 hours.
When to see a doctor
While most healthy people will be able to recover at home, if symptoms get worse, you may need to get medical attention. Seek medical care promptly for severe symptoms, worsening shortness of breath, chest pain, dehydration, or if symptoms occur in those at high risk.
If you need help with flu symptoms, need a flu test, or a flu shot, caring providers are here to help. To get started, find a center near you and walk in or save your spot online.
FAQs
Here are frequently asked questions about H3N2 influenza.
What is the H3N2 flu strain?
H3N2 is a subtype of influenza A identified by two specific proteins, hemagglutinin (H3) and neuraminidase (N2). It is one of the main viruses responsible for the flu this season.
Why is H3N2 significant this flu season?
The H3N2 flu strain mutates quickly, spreads efficiently, and causes more severe illness, particularly in older adults. Flu seasons dominated by H3N2 are often associated with higher hospitalization rates.
What symptoms does H3N2 flu cause?
H3N2 flu strain symptoms are similar to other flu strains and may include fever, chills, cough, sore throat, body aches, headache, fatigue, and congestion.
Is the seasonal flu vaccine effective against H3N2?
The flu vaccine is updated annually to target circulating strains, including H3N2. Effectiveness can vary, but vaccination still provides strong protection against severe illness, hospitalization, and complications.
Who is at the highest risk from the H3N2 flu?
Those at highest risk include adults 65 and older, young children, pregnant women, and people with chronic medical conditions.
How long does H3N2 flu typically last?
Most people recover within 5 to 7 days, though fatigue and cough can linger for a few weeks. Recovery may take longer in older adults or those with underlying health conditions.
References:
- https://www.cdc.gov/fluview/index.html
- https://jamanetwork.com/journals/jama/article-abstract/2843197
- https://www.cdc.gov/cfa-qualitative-assessments/php/data-research/season-outlook25-26-dec-update.html
- https://www.cdc.gov/fluview/surveillance/2025-week-53.html
- https://www.cdc.gov/flu/highrisk/index.htm
